Semiconductor storage device

ABSTRACT

A semiconductor storage device according to the present embodiment includes a plurality of bit lines, a plurality of word lines, and a plurality of memory cells each including a storage element and a switching element which are connected in series between adjacently paired ones of the bit lines. Gates of the switching elements of the memory cells connected between one of the adjacently paired ones of the bit lines are respectively connected to different ones of the word lines. A plurality of the storage elements and a plurality of the switching elements of the adjacent memory cells are alternately connected in series.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/172,739, filed on Feb. 4, 2014 and titled “SEMICONDUCTOR STORAGEDEVICE,” which is a continuation of U.S. application Ser. No.13/424,798, filed on Mar. 20, 2012 and titled “SEMICONDUCTOR STORAGEDEVICE,” which is based upon and claims the benefit of priority from theprior Japanese Patent Application No. 2011-68661, filed on Mar. 25,2011, the entire contents of each of which is hereby incorporated byreference herein.

FIELD

The embodiments of the present invention relate to a semiconductorstorage device.

BACKGROUND

In recent years, development of nonvolatile semiconductor storagedevices that use resistance elements as storage elements such as an MRAM(Magnetoresistive Random Access Memory), a PCRAM (Phase Change RandomAccess Memory), and an RRAM (Resistive Random Access Memory) isunderway.

As for the MRAM among these storage devices, magnetic field writing andspin-transfer torque writing are normally known as data writingtechniques and these techniques are advantageous in high speedprocessing because of the use of the magnetization reversal of magneticbodies. The spin-transfer torque writing has a characteristic such thata spin injection current becomes smaller in an amount necessary formagnetization reversal as magnetic bodies is smaller in size. The amountof a current necessary for spin-transfer magnetization reversal (areversal threshold current) is defined by the density of a currentflowing to a magnetoresistive element (an MTJ element). For this reason,the reversal threshold current decreases as the area of the MTJ elementis reduced. That is, when the area of the MTJ element is reduced, thereversal threshold current is downscaled. Therefore, the MRAM based onthe spin-transfer torque writing is advantageous in high integration,low power consumption, and high performance, and is expected as anonvolatile semiconductor storage device that can replace a DRAM.

However, in conventional memory cell layouts, MTJ elements are notarranged uniformly on a semiconductor substrate and some adjacent MTJelements are arranged at a smaller interval. Therefore, the MRAM hasproblems such that it is more difficult to perform exposure andprocessing in lithographic and etching steps with the progress ofdownscaling.

Meanwhile, there has been known a method of apparently uniformlyarranging MTJ elements on a semiconductor substrate by arranging dummyMTJ elements in the spaces between adjacent MTJ elements. In thismethod, a stable shape is ensured for the MTJ elements because of theuniform arrangement; however, this method disadvantageously createsother problems in processing steps such as the need to selectively andelectrically isolate only the dummy MTJ elements from other electrodes,wirings, and the like. In addition, the memory size cannot be reducedfor the area by which the dummy MTJ elements are arranged. Under thesecircumstances, conventional MRAMs cannot simultaneously meet the demandsof downscaling and mass storage capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a configuration example of amemory cell array MCA in an MRAM according to a first embodiment;

FIG. 2 is an explanatory diagram showing a configuration and anoperation of a single memory cell MC;

FIG. 3 is a timing diagram showing voltages of the word lines WLi andthat of a sense-amplifier enable signal SAEN at the time of a data readoperation;

FIG. 4 is a plan view showing a layout of the memory cell array MCAaccording to the first embodiment;

FIGS. 5A to 5D are cross-sectional views taken along lines A-A, B-B,C-C, and D-D of FIG. 4, respectively;

FIGS. 6A and 6B are plan views showing a layout of the active areas AAand gate electrodes GC and a plan view showing a layout of upperelectrodes UE and via contacts V0;

FIGS. 7A and 7B are plan views showing a layout of the MTJ elements andthe upper electrodes UE and a layout of the upper electrodes UE, viacontacts V1, and the bit lines BL;

FIG. 8 is a schematic diagram showing a configuration example of thememory cell array MCA in an MRAM according to a modification of thefirst embodiment;

FIG. 9 is a timing diagram showing an operation example of the wordlines WLi, sense-amplifier enable signals SAENA and SAENB, and switchenable signals ENA and ENB at the time of a data read operation;

FIG. 10 is a schematic diagram showing a configuration example of thememory cell array MCA in an MRAM according to a second embodiment;

FIG. 11 is a plan view showing a layout of the memory cell array MCAaccording to the second embodiment;

FIGS. 12A and 12B are cross-sectional views taken along lines C-C andD-D of FIG. 11;

FIGS. 13A to 13 c are plan views showing a layout of the upperelectrodes UE and the via contacts V0, a layout of the MTJ elements andthe upper electrodes UE, and a layout of the upper electrodes UE, thevia contacts V1 and the bit lines BL;

FIG. 14 is a schematic diagram showing a configuration example of thememory cell array MCA in an MRAM according to a modification of thesecond embodiment

FIGS. 15A to 15D are cross-sectional views of the memory cell array MCAin an MRAM according to a third embodiment;

FIGS. 16A and 16B are cross-sectional views of the memory cell array MCAin an MRAM according to a fourth embodiment;

FIG. 17 is a schematic diagram showing a configuration example of thememory cell array MCA, local DQ lines LDQ, and the sense amplifiers SAin an MRAM according to a fifth embodiment;

FIG. 18 is a schematic diagram showing a configuration example of thememory cell array MCA, local DQ lines LDQ, and the sense amplifiers SAin an MRAM according to a sixth embodiment;

FIG. 19 is a schematic diagram showing a configuration example of thememory cell array MCA, the local DQ lines LDQ, the transfer gates XFER,and the sense amplifiers SA in an MRAM according to a modification ofthe fifth embodiment; and

FIG. 20 is a schematic diagram showing a configuration example of thememory cell array MCA, the local DQ lines LDQ, the transfer gates XFER,and the sense amplifiers SA in an MRAM according to a modification ofthe sixth embodiment.

DETAILED DESCRIPTION

A semiconductor storage device according to the present embodimentcomprises a plurality of bit lines, a plurality of word lines, and aplurality of memory cells each including a storage element and aswitching element which are connected in series between adjacentlypaired ones of the bit lines. Gates of the switching elements of thememory cells connected between one of the adjacently paired ones of thebit lines are respectively connected to different ones of the wordlines. A plurality of the storage elements and a plurality of theswitching elements of the adjacent memory cells are alternatelyconnected in series.

Embodiments will now be explained with reference to the accompanyingdrawings. The present invention is not limited to the embodiments.

First Embodiment

FIG. 1 is a schematic diagram showing a configuration example of amemory cell array MCA in an MRAM according to a first embodiment. Thememory cell array MCA includes a plurality of memory cells MC arrangedtwo-dimensionally in a matrix. The memory cells MC are arranged tocorrespond to intersections between adjacently paired bit lines BLi andBLi+1, where i is an integer, and word lines WLi, respectively. The bitlines BLi extend in a column direction that serves as a first direction,and the word lines WLi extend in a row direction that serves as a seconddirection.

Each memory cell MC includes a magnetic tunnel junction (MTJ) elementand a cell transistor CT as a switching element connected in seriesbetween the paired bit lines BLi and BLi+1. Gates of the celltransistors CT of the memory cells MC connected between the paired bitlines BLi and BLi+1 are connected to different word lines WLi,respectively. For example, the gates of the cell transistors CT of firstto sixth memory cells MC0 to MC5 are connected to word lines WL0 to WL5,respectively.

Furthermore, in the first embodiment, the MTJ elements of the firstmemory cell MC0, among the memory cells MCi connected between the pairedbit lines BLi and BLi+1, is connected to one end (drain or source) ofthe cell transistor CT of the second memory cell MC1 adjacent to thefirst memory cell MC0. The MTJ element of the second memory cell MC1 isconnected to one end (drain or source) of the cell transistor CT of thethird memory cell MC2 further adjacent to the second memory cell MC1.That is, the MTJ element of one memory cell MCi is connected to one end(drain or source) of the cell transistor CT of the memory cell MCi+1further adjacent to the memory cell MCi. In this manner, in the firstembodiment, the memory cells MCi connected between the adjacently pairedbit lines BLi and BLi+1 are connected between the same paired bit linesBLi and BLi+1 and configured so that the MTJ elements and the celltransistors CT are alternately connected in series. That is, between theadjacently paired bit lines BLi and BLi+1, the MTJ elements and the celltransistors CT are alternately connected in series in the order of, forexample, CT, MTJ, CT, MTJ, CT, MTJ, CT, MTJ, and the like.

Further, one end of the cell transistor CT of each memory cell MCi isconnected to the bit line BLi or BLi+1 alternately in the order of i andi+1. That is, one end of the cell transistor CT of the memory cell MC0is connected to the bit line BL0, that of the cell transistor CT of thememory cell MC1 is connected to the bit line BL1, that of the celltransistor CT of the memory cell MC2 is connected to the bit line BL0,and that of the cell transistor CT of the memory cell MC3 is connectedto the bit line BL1.

One of each pair of bit lines BLi and BLi+1, that is, the bit line BLiis connected to a sense amplifier SA, and the other bit line BLi+1 isconnected to a reference potential VSS (for example, ground). The senseamplifier SA carries a current to each MTJ element via the bit line BLiand compares a cell current Icell flowing to the MTJ element with areference current Iref. The sense amplifier SA thereby detects a logicalstate of data stored in the memory cell MCi.

Currents flowing to the MTJ elements of the two memory cells MCi andMCi+1 adjacent in the row direction are opposite in a current flowdirection. For example, the current flowing to the memory cell MC0 is inthe direction from the cell transistor CT to the MTJ element (see thearrow A2 shown in FIG. 2). The current flowing to the memory cell MC1 isin the direction from the MTJ element to the cell transistor CT (see thearrow A1 shown in FIG. 2). Therefore, in the first embodiment, each ofthe sense amplifiers SA needs to detect the currents flowing to thememory cells MC in the two directions. In this case, it suffices toconnect each of the sense amplifiers SA to one of the paired bit linesBLi and BLi+1. This can make the connection arrangement between thesense amplifiers SA and the bit lines BL simpler. It also suffices toprovide the sense amplifiers SA for every pair of bit lines BLi andBLi+1. This can decrease the number of the sense amplifiers SA. Thiscontributes to reducing the chip size of the MRAM.

FIG. 2 is an explanatory diagram showing a configuration and anoperation of a single memory cell MC. Each memory cell MC includes theMTJ element serving as a storage element and the cell transistor CT. TheMTJ element and the cell transistor CT are connected in series betweenthe two bit lines BLi and BLi+1. In FIG. 2, the cell transistor CT isarranged near the bit line BLi and the MTJ element is arranged near thebit line BLi+1. However, there are cases where the MTJ element isarranged near the bit line BLi and the cell transistor CT is arrangednear the bit line BLi+1 as in the case of the memory cells MC1, MC3, andMC5 shown in FIG. 1. The gate of the cell transistor CT is connected toone word line WL.

The MTJ element that uses the TMR (tunneling magnetoresistive) effecthas a stacked structure in which a nonmagnetic layer (tunnel dielectricfilm) is sandwiched between two ferromagnetic layers. The MTJ elementstores therein digital data by a change in a magnetic resistance due tothe spin-polarized tunneling. The MTJ element can be set in a lowresistance state or a high resistance state depending on magnetizationorientations of the two ferromagnetic layers. For example, when it isdefined that the low resistance state indicates data “0” and that thehigh resistance state indicates data “1”, one-bit data can be recordedin the MTJ element. Alternatively, it can be defined that the lowresistance state indicates data “1” and that the high resistance stateindicates data “0”. For example, the MTJ element is configured tosequentially stack a pinned layer P, a tunnel barrier layer, and arecording layer F. The pinned layer P and the recording layer F are madeof a ferromagnetic body and the tunnel barrier layer is an insulatingfilm. The pinned layer P has a fixed magnetization orientation. Therecording layer F has a variable magnetization orientation. The MTJelement stores therein data depending on the magnetization orientationof the recording layer F.

At the time of a data write operation, when a current equal to or higherthan a write threshold current flows to the MTJ element in an arrow A1direction, the magnetization orientation of the recording layer Fbecomes anti-parallel to that of the pinned layer P. The MTJ elementthereby turns into the high resistance state (data “1”). At the time ofthe data write operation, when the current equal to or higher than thewrite threshold current flows to the MTJ element in an arrow A2direction, the magnetization orientation of the recording layer Fbecomes parallel to that of the pinned layer P. The MTJ element therebyturns into the low resistance state (data “0”). In this manner,different data can be written to the MTJ element depending on thecurrent flow direction.

FIG. 2 shows an example in which the pinned layer P is connected to thebit line BLi+1 and the recording layer F is connected to the celltransistor CT. Conversely, the memory cell MC can be configured so thatthe recording layer F is connected to the bit line BLi+1 and that thepinned layer P is connected to the cell transistor CT. In the lattercase, the polarity of the written data is reversed with respect to thatshown in FIG. 2.

At the time of a data read operation, when a read voltage is applied tothe memory cells MC0, MC2, and MC4 in the arrow A2 direction, the cellcurrent Icell in response to the resistance state of the MTJ element ineach of the memory cells MC0, MC2, and MC4 flows to the bit line BL0.The sense amplifier SA detects the logical state of the MTJ element bycomparing the cell current Icell with the reference current Iref.

Furthermore, when the read voltage is applied to the memory cells MC1,MC3, and MC5 in the arrow A1 direction, the cell current Icell inresponse to the resistance state of the MTJ element in each of thememory cells MC1, MC3, and MC5 flows to the bit line BL0. The senseamplifier SA detects the logical state of the MTJ element by comparingthe cell current Icell with the reference current Iref.

FIG. 3 is a timing diagram showing voltages of the word lines WLi andthat of a sense-amplifier enable signal SAEN at the time of a data readoperation. For example, the word lines WL0 to WL3 are activated in thisorder. The sense-amplifier enable signal SAEN is activated at the timingof the activation of the word lines WL0 to WL3. Preferably, thesense-amplifier enable signal SAEN is activated after the cell currentIcell becomes stable following the activation of the word lines WL0 toWL3.

With this configuration, at a time t0, the sense amplifiers SA inrespective columns detect data stored in the memory cells MC connectedto the word line WL0 via the bit lines BL0, BL2, BL4, BL6, and the like.At a time t1, the sense amplifiers SA in the respective columns detectdata stored in the memory cells MC connected to the word line WL1 viathe bit lines BL0, BL2, BL4, BL6, and the like. At a time t2, the senseamplifiers SA in the respective columns detect data stored in the memorycells MC connected to the word line WL2 via the bit lines BL0, BL2, BL4,BL6, and the like. Further, at a time t3, the sense amplifiers SA in therespective columns detect data stored in the memory cells MC connectedto the word line WL3 via the bit lines BL0, BL2, BL4, BL6, and the like.Thereafter, the sense amplifiers SA detect data stored in the memorycells MC connected to the word lines WL4, WL5, and the like in asequential manner.

By appropriately and selectively driving the word lines WLi, it ispossible to read data stored in the arbitrary memory cell MC among thememory cells MC0 to MC5 adjacent in the column direction via the samepaired bit lines BL0 and BL1.

While the data read operation has been described above, the data writeoperation can be performed by carrying a write current to the memorycells MC in the direction according to write data as described withreference to FIG. 2.

(Layout)

FIG. 4 is a plan view showing a layout of the memory cell array MCAaccording to the first embodiment. FIGS. 5A to 5D are cross-sectionalviews taken along lines A-A, B-B, C-C, and D-D of FIG. 4, respectively.FIG. 6A is a plan view showing a layout of the active areas AA and gateelectrodes GC (word lines WL), and FIG. 6B is a plan view showing alayout of upper electrodes UE and via contacts V0. FIG. 7A is a planview showing a layout of the MTJ elements and the upper electrodes UE,and FIG. 7B is a plan view showing a layout of the upper electrodes UE,via contacts V1, and the bit lines BL. Although FIGS. 6A to 7B do notshow contact plugs CB0 and CB1, FIGS. 5A to 5D show that the contactplugs CB0 and CB1 are arranged in lower portions of both the viacontacts V0 and the MTJ elements.

With reference to FIG. 4, the memory cells MC connected to paired bitlines BL2 and BL3 and those to paired bit lines BL4 and BL5 are the samein configurations as the memory cells MC connected to the paired bitlines BL0 and BL1. That is, the configuration of the memory cells MCconnected to the paired bit lines BL0 and BL1 is repeatedly formed inthe row direction. Therefore, the configuration of the memory cells MCconnected to the paired bit lines BL0 and BL1 is described and those ofthe memory cells MC connected to the other pairs of bit lines BL are notdescribed herein.

A plurality of active areas AA are isolated from one another by STI(Shallow Trench Isolation) to correspond to the memory cells MC on asemiconductor substrate 10. That is, diffusion layers (source and drain)of the cell transistor CT are provided individually for every memorycell MC and not shared with the cell transistors CT of the adjacentmemory cells MC. The active areas AA are arranged in the columndirection serving as the first direction and constitute active areastrings in respective columns. In the two active area strings adjacentin the row direction serving as the second direction and substantiallyorthogonal to the column direction, the active areas AA are arranged ina half-pitch-staggered (or a half-pitch-shifted) shape in the columndirection (see FIG. 6A). In other words, the active areas AA arearranged in a half-pitch-staggered shape to correspond to the respectivebit lines BLi because the active area strings correspond to the bitlines BLi, respectively. This can be understood from the comparison ofFIG. 5A with FIG. 5B. In this manner, in the plan layout, the activeareas AA are arranged in a staggered shape.

As shown in FIGS. 5A and 5B, one cell transistor CT is formed in each ofthe active areas AA. The cell transistor CT includes the gate electrodeGC and diffusion layers 20 and 21 formed on both sides of a channelunder the gate electrode GC. One of the diffusion layers 20 and 21 ofthe cell transistor CT, that is, the diffusion layer 21 is connected toa lower end of the MTJ element via the contact plug CB1.

As shown in FIGS. 4, 5C, and 5D, the upper electrodes UE each having oneend connected to an upper end of the MTJ element extend in the rowdirection. As shown in FIG. 5C, for example, the other end of the upperelectrode UE having one end connected to the MTJ element provided belowthe bit line BL1 is connected to the diffusion layer 20 of the celltransistor CT provided below the bit line BL0 adjacent to the bit lineBL1 via the via contact V0 and the contact plug CB0. As shown in FIG.5D, the other end of the upper electrode UE having one end connected tothe MTJ element provided below the bit line BL0 is connected to thediffusion layer 20 of the cell transistor CT provided below the bit lineBL1 adjacent to the bit line BL0 via the via contact V0 and the contactplug CB0. Furthermore, as shown in FIGS. 5C, 5D, and 7B, the other endof the upper electrode UE having one end connected to the MTJ elementprovided below the bit line BL0 is connected to the bit line BL1 via thevia contact V1 provided on the upper electrode UE.

As shown in FIGS. 4 and 5A to 5D, the active areas AA arehalf-pitch-staggered for every bit line BLi. Therefore, although theupper electrodes UE are not staggered in the row direction, each of theupper electrodes UE can electrically connect the upper end of the MTJelement present below the bit line BL0 to the bit line BL1 adjacent tothe bit line BL0 and to the diffusion layer 20 of the cell transistor CTpresent below the bit line BL1.

As shown in FIGS. 4 and 6A, each of the active areas AA has a shapeextending in the column direction (a shape of a rectangle or anellipse). As shown in FIGS. 6B and 7A, each of the upper electrodes UEhas a shape extending in the row direction (a shape of a rectangle or anellipse). As viewed from above a surface of the semiconductor substrate10, the active areas AA and the upper electrodes UE are alternately andcontinuously arranged so that one end of each of the active areas AA orthe upper electrode UE overlaps with the other end of the upperelectrode UE or the active area AA. These overlapping portionscorrespond to the portions indicated by MTJ, V0, and V1 in FIG. 4.Moreover, in the first embodiment, ends of the upper electrodes UEadjacent in the column direction are arranged not in a staggered shapebut uniformly in the row direction as shown in FIGS. 6B and 7A.Therefore, as viewed from above the surface of the semiconductorsubstrate 10, a plurality of active areas AA and a plurality of upperelectrodes UE corresponding to the paired bit lines BL0 and BL1 appearto be arranged to be of a rectangular wave shape continuous in the rowdirection. The configuration of the memory cell array MCA according tothe first embodiment is hereinafter also referred to as “rectangularwave configuration”.

The layout described above can realize an equivalent circuit to thememory cell array MCA as shown in FIG. 1.

While the example in which the bit lines BL and the word lines WL arearranged at the same pitch has been described above, the pitch of thebit lines BL can be set different from that of the word lines WL.

In the first embodiment, the active areas AA, the memory cells MC, theMTJ elements, the cell transistors CT, and the via contacts V0 and V1are arranged in a staggered shape on the surface of the semiconductorsubstrate 10. The constituent elements including the MTJ elements arethereby arranged uniformly on the surface of the semiconductor substrate10. Similarly, in cut portions of the active areas AA adjacent in thecolumn direction, the constituent elements including the MTJ elementsare also arranged in a staggered shape.

Furthermore, the two most adjacent MTJ elements are arranged obliquely(almost at 45 degrees) with respect to the row and column directions soas to correspond to the different bit lines BL and different word linesWL, respectively. For example, when the layout pitch of the bit lines BLis equal to that of the word lines WL, the layout angle between the mostadjacent MTJ elements is 45 degrees. Therefore, a layout pitch D_(MTJ)of the two adjacent MTJ elements can be set to √2×L, where L is a layoutpitch (Line and Space pitch) of the bit lines BL (the word lines WL).Providing that L is 2F, where F=Minimum Feature Size, the layout pitchD_(MTJ) can be relaxed (widen) to √2 times as large as L or 2F, that is,√2×L or 2√2×F. This can facilitate performing lithographic and shapeprocesses on the MTJ elements in the manufacturing of the MRAM accordingto the first embodiment. Similarly, the two most adjacent via contactsV0 (or V1) are arranged obliquely (almost at 45 degrees) with respect tothe row and column directions so as to correspond to the different bitlines BL and different word lines WL, respectively. Therefore, similarlyto the MTJ elements, the layout pitch of the two adjacent via contactsV0 (or V1) can be set to √2×L. This can facilitate performinglithographic and shape processes on the via contacts V0 (or V1) in themanufacturing of the MRAM according to the first embodiment. As aresult, the MRAM can be further downscaled and the storage capacity ofthe MRAM can be further increased accordingly.

In the first embodiment, as shown in FIGS. 5A to 5D, the via contacts V1serving as first upper contacts are not formed above the MTJ elementsbut provided above the via contacts V0 serving as lower contacts via theupper electrodes UE. This can protect the MJT elements from processdamage during the formation of the via contacts V1.

Modification of First Embodiment

FIG. 8 is a schematic diagram showing a configuration example of thememory cell array MCA in an MRAM according to a modification of thefirst embodiment. In the first embodiment, the cell currents Icellflowing to the MTJ elements MC of the two memory cells MCi and MCi+1adjacent in the row direction at the time of a data read operation areopposite in the current flow direction (A1 and A2 shown in FIG. 2). Inthe MRAM according to the present modification, the cell currents Icellflowing to the MTJ elements of a plurality of memory cells MC at thetime of a data read operation are identical in the current flowdirection (only A1 or A2 shown in FIG. 2). Therefore, in the presentmodification, the sense amplifiers SA are provided to correspond to eachof the bit lines BLi on both sides of the memory cell array MCA,respectively. The sense amplifiers SA provided on the left side of thememory cell array MCA are denoted by SAL and those on the right side aredenoted by SAR in the present modification.

Furthermore, switching elements SWAL, SWBL, SWAR, and SWAL are providedto switch over the connections between the sense amplifiers SAL and SARand the reference potential VSS. The memory cell array MCA according tothe present modification can be configured similarly (rectangular waveconfiguration) to that according to the first embodiment.

The switching elements SWAL are provided between the bit lines BL1, BL3,BL5, and BL7 and the reference potential VSS, respectively. Theswitching elements SWBL are provided between the bit lines BL1, BL3,BL5, and BL7 and the sense amplifiers SAL, respectively. The switchingelements SWAR are provided between the bit lines BL0, BL2, BL4, and BL6and the sense amplifiers SAR, respectively. The switching elements SWBRare provided between the bit lines BL0, BL2, BL4, and BL6 and thereference potential VSS, respectively.

FIG. 9 is a timing diagram showing an operation example of the wordlines WLi, sense-amplifier enable signals SAENA and SAENB, and switchenable signals ENA and ENB at the time of a data read operation.

For example, the word lines WL0 to WL3 are activated in this order. Theswitch enable signal ENA and the sense-amplifier enable signal SAENA areactivated at the timing of the activation of the word lines WL0 and WL2.The switch enable signal ENB and the sense-amplifier enable signal SAENBare activated at the timing of the activation of the word lines WL1 andWL3.

At t0 and t2, the switch enable signal ENA is activated, whereby the bitlines BL0, BL2, BL4, and BL6 are connected to the sense amplifiers SAR,respectively and the bit lines BL1, BL3, BL5 and BL7 are connected tothe reference potential VSS. The sense amplifiers SAR operates inresponse to the activation of the sense-amplifier enable signal SANEA.The cell current Icell thereby flows to the memory cells MC connected tothe word lines WL0 and WL2. The current flow direction of the cellcurrent Icell is A2 shown in FIG. 2.

At t1 and t3, the switch enable signal ENB is activated, whereby the bitlines BL1, BL3, BL5, and BL7 are connected to the sense amplifiers SAL,respectively and the bit lines BL0, BL2, BL4 and BL6 are connected tothe reference potential VSS. The sense amplifiers SAL operates inresponse to the activation of the sense-amplifier enable signal SANEB.The cell current Icell thereby flows to the memory cells MC connected tothe word lines WL1 and WL3. The current flow direction of the cellcurrent Icell is A2 shown in FIG. 2 similarly to that at t0 and t2.

In this manner, the cell current Icell flows to the memory cells MC inthe same direction, which eliminates variations in the channelresistances of the cell transistors CT due to, for example, thesubstrate bias effect. This can increase a read margin (signaldifference between “1” and “0”).

Because the sense amplifiers SAL and SAR are provided to correspond tothe respective bit lines BL, the number of arranged sense amplifiers SALand SAR, that is, the area by which the sense amplifiers SAL and SAR areformed is larger than that according to the first embodiment. However,because the configuration of the memory cell array MCA according to thepresent modification is identical to that of the memory cell array MCAaccording to the first embodiment, this modification can achieve othereffects of the first embodiment.

Second Embodiment

FIG. 10 is a schematic diagram showing a configuration example of thememory cell array MCA in an MRAM according to a second embodiment. Thesecond embodiment differs from the first embodiment in the arrangementof the memory cells MC. In the first embodiment, as viewed from abovethe surface of the semiconductor substrate 10, the active areas AA andthe upper electrodes UE appear to be arranged to be of the rectangularwave shape continuous in the row direction as shown in FIG. 4. In thesecond embodiment, by contrast, as viewed from above the surface of thesemiconductor substrate 10, the upper electrodes UE appear to bearranged to be of a staircase shape continuous in an oblique directionwith respect to the row and column directions as shown in FIG. 11. Theconfiguration of the memory cell array MCA according to the secondembodiment is also referred to as “staircase configuration”.

The second embodiment is identical to the first embodiment in thearrangement of the bit lines BLi and the word lines WLi and theconfiguration of each memory cell MC. Although the arrangement of thebit lines BLi is the same, the bit lines BL0 to BL6 according to thesecond embodiment are denoted differently from the first embodiment forthe sake of convenience.

More specifically, the MTJ element of the first memory cell MC0 among aplurality of memory cells MCi is connected to one end (drain or source)of the cell transistor CT of the second memory cell MC1 adjacent to thefirst memory cell MC0. The MTJ element of the second memory cell MC1 isconnected to one end of the cell transistor CT of the third memory cellMC2 further adjacent to the first memory cell MC0. That is, the MTJelement of one memory cell MCi is connected to one end of the celltransistor CT of the memory cell MCi+1 further adjacent to the memorycell MCi.

Furthermore, in the second embodiment, the first memory cell MC0 isconnected between the first bit line BL0 and the second bit line BL1adjacent to the first bit line BL0. The second memory cell MC1 isconnected between the second bit line BL1 and the third bit line BL2adjacent to the second bit line BL1. The third memory cell MC2 isconnected between the third bit line BL2 and the fourth bit line BL3adjacent to the third bit line BL2. That is, one memory cell MCi isconnected between the bit line BLi and the bit line BLi+1 adjacent tothe bit line BLi.

As described above, in the second embodiment, when the bit lines BLi arearranged in parallel to the bit lines BLi+1 adjacent to the bit linesBLi, the MTJ elements and the cell transistors CT of the memory cellsMCi are alternately connected in series. That is, when the bit lines BLiare arranged in parallel to the adjacent bit lines BLi+1, the MTJelements and the cell transistors CT are alternately connected in seriesin the order of, for example, CT, MTJ, CT, MTJ, and the like. In thismanner, the memory cells MCi connected in series are connected betweendifferent pairs of bit lines BLi and BLi+1, and connected to thedifferent word lines WLi, respectively.

The MRAM according to the second embodiment can be configured similarlyto that according to the first embodiment in other aspects. Therefore,each of the sense amplifiers SA detects currents flowing to the memorycells MC in two directions. In this case, it suffices to connect each ofthe sense amplifiers SA to one of the pair bit lines BLi and BLi+1. Thiscan make the connection arrangement between the sense amplifiers SA andthe bit lines BL simpler. It also suffices to provide the senseamplifiers SA for every pair of bit lines BLi and BLi+1. This candecrease the number of the sense amplifiers SA.

The same timing diagram as that shown in FIG. 3 can be used as thatshowing the voltages of the word lines WLi and that of thesense-amplifier enable signal SAEN at the time of the data readoperation.

Therefore, at a time t0, the sense amplifiers SA in the respectivecolumns detect data stored in the memory cells MC connected to the wordline WL0 via the bit lines BL1, BL3, BL5, and the like. At this time,the paired bit lines BL connected to the both sides of the read-targetmemory cells MC are (BL0, BL1), (BL2, BL3), (BL4, BL5), and the like. Ata time t1, the sense amplifiers SA in the respective columns detect datastored in the memory cells MC connected to the word line WL1 via the bitlines BL1, BL3, BL5, and the like. At this time, the paired bit lines BLconnected to the both sides of the read-target memory cells MC are (BL1,BL2), (BL3, BL4), (BL5, BL6), and the like. At a time t2, the senseamplifiers SA in the respective columns detect data stored in the memorycells MC connected to the word line WL2 via the bit lines BL1, BL3, BL5,and the like. At this time, the paired bit lines BL connected to theboth sides of the read-target memory cells MC are (BL0, BL1), (BL2,BL3), (BL4, BL5), and the like. At a time t3, the sense amplifiers SA inthe respective columns detect data stored in the memory cells MCconnected to the word line WL3 via the bit lines BL1, BL3, BL5, and thelike. At this time, the paired bit lines BL connected to the both sidesof the read-target memory cells MC are (BL1, BL2), (BL3, BL4), (BL5,BL6), and the like. When the sense amplifiers SA detect data stored inthe memory cells MC connected to the word lines WL4, WL5, and the like,the paired bit lines BL are alternately changed between (BL0, BL1),(BL2, BL3), (BL4, BL5), and the like and (BL1, BL2), (BL3, BL4), (BL5,BL6), and the like, similarly to the above.

For example, when the data is read from or written to the memory cellMC0, the word line WL0 is activated. The sense amplifier SA reads thedata from the memory cell MC0 via the paired bit lines BL0 and BL1 orcarries a write current to the memory cell MC0 in the directionaccording to the write data via the paired bit lines BL0 and BL1.

When the data is read from the memory cell MC1, the word line WL1 isactivated. The sense amplifier SA reads the data from the memory cellMC1 via the paired bit lines BL1 and BL2.

When the data is read from the memory cell MC2, the word line WL2 isactivated. The sense amplifier SA reads the data from the memory cellMC2 via the paired bit lines BL2 and BL3.

As described above, in the MRAM according to the second embodiment, byappropriately and selectively driving the word lines WLi, it is possibleto read data stored in the arbitrary memory cell MC among the memorycells MC0 to MC5 connected in series. At this time, the data read fromthe memory cells MC0 to MC5 is transmitted to the sense amplifiers SAvia the corresponding paired bit lines BL0 and BL1, BL1 and BL2, BL2 andBL3, BL3 and BL4, BL4 and BL5, and BL5 and BL6, respectively. Moreover,in the MRAM according to the second embodiment, by appropriately andselectively driving the word lines WLi, it is possible to write data tothe arbitrary memory cell MC among the memory cells MC0 to MC5. At thistime, the data is written to the memory cells MC0 to MC5 via thecorresponding paired bit lines BL0 and BL1, BL1 and BL2, BL2 and BL3,BL3 and BL4, BL4 and BL5, and BL5 and BL6, respectively.

(Layout)

FIG. 11 is a plan view showing a layout of the memory cell array MCAaccording to the second embodiment. FIGS. 12A and 12B arecross-sectional views taken along lines C-C and D-D of FIG. 11,respectively. Cross-sectional views taken along lines A-A and B-B ofFIG. 11 are not shown herein because these are substantially identicalto FIGS. 5A and 5B, respectively. FIG. 13A is a plan view showing alayout of the upper electrodes UE and the via contacts V0. FIG. 13B is aplan view showing a layout of the MTJ elements and the upper electrodesUE. FIG. 13C is a plan view showing a layout of the upper electrodes UE,the via contacts V1, and the bit lines BL. The layout view of the activeareas AA and the word lines WL is not shown herein because it issubstantially identical to FIG. 6A. The contact plugs CB0 and CB1 areidentical to those according to the first embodiment.

The second embodiment differs from the first embodiment in that theupper electrodes UE adjacent in the column direction arehalf-pitch-staggered in the row direction. Layouts of other constituentelements in the second embodiment are identical to those ofcorresponding elements of the first embodiment.

With reference to FIGS. 13A to 13C, it can be understood that the upperelectrodes UE adjacent in the column direction are half-pitch-staggered(half-pitch-shifted) in the row direction. That is, in the plan layout,like the active areas AA, the upper electrodes UE are arranged in astaggered shape. Similarly, in the cut portions of the upper electrodesUE adjacent in the row direction, the upper electrodes UE are arrangedin a staggered shape.

As shown in FIGS. 11, 12A, and 12B, the upper electrodes UE each havingone end connected to the upper end of the MTJ element extend in the rowdirection. For example, as shown in FIG. 12A, the other end of the upperelectrode UE having one end connected to the MTJ element provided belowthe bit line BL1 is connected to the diffusion layer 20 of the celltransistor CT provided below the bit line BL2 adjacent to the bit lineBL1 via the via contact V0 and the contact plug CB0. As shown in FIG.12B, the other end of the upper electrode UE having one end connected tothe MTJ element provided below the bit line BL0 is connected to thediffusion layer 20 of the cell transistor CT provided below the bit lineBL1 adjacent to the bit line BL0 via the via contact V0 and the contactplug CB0. Furthermore, the other end of the upper electrode UE isconnected to the bit line BL1 via the via contact V1 provided on theupper electrode UE as shown in FIGS. 12A and 12B.

As shown in FIGS. 11, 12A, and 12B, the active areas AA adjacent in therow direction are half-pitch-staggered in the column direction, and theupper electrode UE adjacent in the column direction arehalf-pitch-staggered in the row direction. As shown in FIG. 11, each ofthe active areas AA has a shape extending in the column direction (theshape of the rectangle or ellipse). Each of the upper electrodes UE hasa shape extending in the row direction (the shape of the rectangle orellipse). As viewed from above the surface of the semiconductorsubstrate 10, the active areas AA and the upper electrodes UE arealternately and continuously arranged so that one end of each of theactive areas AA or upper electrode UE overlaps with the other end of theupper electrode UE or the active area AA. These overlapping portionscorrespond to the portions indicated by MTJ, V0, and V1 in FIG. 11.Therefore, as viewed from above the surface of the semiconductorsubstrate 10, a plurality of active areas AA and a plurality of upperelectrodes UE appear to be arranged to be of a staircase shapecontinuous in the oblique direction at about 45 degrees with respect tothe row and column directions.

The layout described above can realize an equivalent circuit to thememory cell array MCA as shown in FIG. 10.

While the example in which the bit lines BL and the word lines WL arearranged at the same pitch has been described above, the pitch of thebit lines BL can be set different from that of the word lines WL.

In the second embodiment, like the first embodiment, the constituentelements including the MTJ elements are arranged uniformly on thesurface of the semiconductor substrate 10. The arrangement of the MTJelements according to the second embodiment is identical to thataccording to the first embodiment. Therefore, the second embodiment canalso achieve effects of the first embodiment.

Modification of Second Embodiment

FIG. 14 is a schematic diagram showing a configuration example of thememory cell array MCA in an MRAM according to a modification of thesecond embodiment. In the present modification, the modification of thefirst embodiment is applied to the second embodiment.

In the second embodiment, the cell currents Icell flowing to the MTJelements of the two memory cells MCi and MCi+1 in the adjacent columnsat the time of a data read operation are opposite in the current flowdirection. In the MRAM according to the present modification, the cellcurrents Icell flowing to the MTJ elements of a plurality of memorycells MC at the time of a data read operation are identical in thecurrent flow direction. Therefore, in the present modification, thesense amplifiers SA are provided to correspond to each of the bit linesBLi on the both side of the memory cell array MCA, respectively. Thesense amplifiers SA provided on the left side of the memory cell arrayMCA are denoted by SAL and those on the right side are denoted by SAR inthe present modification.

Furthermore, the switching elements SWAL, SWBL, SWAR, and SWAL areprovided to switch over the connections between the sense amplifiers SALand SAR and the reference potential VSS. The memory cell array MCAaccording to the present modification can be configured similarly(staircase configuration) to that according to the second embodiment.

The switching elements SWAL are provided between the bit lines BL1, BL3,and BL5 and the reference potential VSS, respectively. The switchingelements SWBL are provided between the bit lines BL1, BL3, and BL5 andthe sense amplifiers SAL, respectively. The switching elements SWAR areprovided between the bit lines BL0, BL2, BL4, and BL6 and the senseamplifiers SAR, respectively. The switching elements SWBR are providedbetween the bit lines BL0, BL2, BL4, and BL6 and the reference potentialVSS, respectively.

The same timing diagram as that shown in FIG. 9 can be used as thatshowing operations for the word lines WLi, sense-amplifier enablesignals SAENA and SAENB, and switch enable signals ENA and ENB at thetime of the data read operation.

For example, at t0 and t2, the switch enable signal ENA is activated,whereby the bit lines BL0, BL2, BL4, and BL6 are connected to the senseamplifiers SAR, respectively and the bit lines BL1, BL3, BL5 and BL7 areconnected to the reference potential VSS. The sense amplifiers SARoperates in response to the activation of the sense-amplifier enablesignal SANEA. The cell current Icell thereby flows to the memory cellsMC connected to the word lines WL0 and WL2. The current flow directionof the cell current Icell is A2 shown in FIG. 2. The paired bit lines BLare (BL0, BL1), (BL2, BL3), (BL4, BL5), and the like.

At t1 and t3, the switch enable signal ENB is activated, whereby the bitlines BL1, BL3, BL5, and BL7 are connected to the sense amplifiers SAL,respectively and the bit lines BL0, BL2, BL4 and BL6 are connected tothe reference potential VSS. The sense amplifiers SAL operates inresponse to the activation of the sense-amplifier enable signal SANEB.The cell current Icell thereby flows to the memory cells MC connected tothe word lines WL1 and WL3. The current flow direction of the cellcurrent Icell is A2 shown in FIG. 2 similarly to that at t0 and t2. Thepaired bit lines BL are (BL1, BL2), (BL3, BL4), (BL5, BL6), and thelike.

In this manner, the cell current Icell flows to the memory cells MC inthe same direction, which eliminates variations in the channelresistances of the cell transistors CT due to, for example, thesubstrate bias effect. This can increase a read margin (signaldifference between “1” and “0”).

Because the sense amplifiers SAL and SAR are provided to correspond tothe respective bit lines BL, the number of arranged sense amplifiers SALand SAR, that is, the area by which the sense amplifiers SAL and SAR areformed is larger than that according to the second embodiment. However,because the configuration of the memory cell array MCA according to thepresent modification is identical to that of the memory cell array MCAaccording to the second embodiment, this modification can achieve othereffects of the second embodiment.

Third Embodiment

FIGS. 15A to 15D are cross-sectional views of the memory cell array MCAin an MRAM according to a third embodiment. The third embodiment differsfrom the first embodiment in that the via contacts V1 are provided abovethe MTJ elements via the upper electrodes UE. The MRAM according to thethird embodiment can be configured similarly to that according to thefirst embodiment in other aspects.

Therefore, FIGS. 15A to 15D correspond to the cross-sectional viewstaken along the lines A-A, B-B, C-C, and D-D of FIG. 4, respectively.However, the via contacts V1 do not overlap with the via contacts V0 butwith the MTJ elements.

An equivalent circuit to the MRAM according to the third embodiment isthe same as FIG. 1 even if the via contacts V1 are provided above theMTJ elements. Therefore, the third embodiment can achieve effectsidentical to those of the first embodiment.

Furthermore, in the third embodiment, the parasitic resistances betweenthe MTJ elements and the bit lines BL can be reduced by as much as thewiring resistances of the upper electrodes UE as compared with the firstembodiment because the via contacts V1 are provided above the MTJelements. The reduction in the parasitic resistances between the MTJelements and the bit lines BL can improve the read margin because theresistances of the MTJ elements are detected as data in the MRAMaccording to the third embodiment.

Fourth Embodiment

FIGS. 16A and 16B are cross-sectional views of the memory cell array MCAin an MRAM according to a fourth embodiment. The fourth embodimentdiffers from the second embodiment in that the via contacts V1 areprovided above the MTJ elements via the upper electrodes UE. The MRAMaccording to the fourth embodiment can be configured similarly to thataccording to the second embodiment in other aspects.

Therefore, FIGS. 16A and 16B correspond to the cross-sectional viewstaken along the lines C-C and D-D of FIG. 11, respectively. However, thevia contacts V1 do not overlap with the via contacts V0 but with the MTJelements. Cross-sectional views taken along lines A-A and B-B of FIG. 11are not shown herein because these are identical to FIGS. 15A and 15B,respectively.

An equivalent circuit to the MRAM according to the fourth embodiment isthe same as FIG. 10 even if the via contacts V1 are provided above theMTJ elements. Therefore, the fourth embodiment can also achieve effectsidentical to those of the second embodiment.

Furthermore, in the fourth embodiment, the parasitic resistances betweenthe MTJ elements and the bit lines BL can be reduced by as much as thewiring resistances of the upper electrodes UE as compared with thesecond embodiment because the via contacts V1 are provided above the MTJelements. The reduction in the parasitic resistances between the MTJelements and the bit lines BL can improve the read margin because theresistances of the MTJ elements are detected as data in the MRAMaccording to the fourth embodiment.

Fifth Embodiment

FIG. 17 is a schematic diagram showing a configuration example of thememory cell array MCA, local DQ lines LDQ, and the sense amplifiers SAin an MRAM according to a fifth embodiment. In the fifth embodiment, aplurality of memory cells MC corresponding to n pairs of bit lines BLiand BLi+1, where n is an integer, are defined as one column block CB.The local DQ lines LDQ and the sense amplifiers SA are configured to beable to simultaneously access n-bit data for every column block CB. Thememory cell array MCA can be configured similarly (rectangular waveconfiguration) to the memory cell array MCA according to the firstembodiment.

For example, in FIG. 17, the memory cells MC corresponding to four pairsof bit lines BLi and BLi+1 constitute each of column blocks CB(N−1),CB(N), CB(N+1), and the like, where N is an integer (hereinafter, alsosimply “column blocks CB”). Four sense amplifiers SA_A0 to SA_A3 andfour sense amplifiers SA_B0 to SA_B3 (hereinafter, also simply “senseamplifiers SA”) are arranged on the both sides of the memory cell arrayMCA, respectively.

Local DQ lines LDQ_A0 to LDQ_A3 and LDQ_B0 to LDQ_B3 (hereinafter, alsosimply “local DQ lines LDQ”) and transfer gates XFER_A and XFER_B(hereinafter, also simply “transfer gates XFER”) are provided betweenthe sense amplifiers SA and the column blocks CB.

Each of the transfer gates XFER is connected between one bit line BL andone local DQ line LDQ, and controlled to be turned on or off by a columnselect line CSL. When the corresponding column block CB is selected,each of the transfer gates XFER connects the bit line BL in the selectedcolumn block CB to the sense amplifier SA via the local DQ line LDQ.

The local DQ lines LDQ are configured to be able to connect the transfergates XFER to the sense amplifiers SA, respectively, or to connect thetransfer gates XFER to the reference voltage VSS (a ground potential).The reference voltage VSS is applied by clamp circuits CLAMP_A andCLAMP_B. The sense amplifiers SA_Ai and the clamp circuit CLAMP_B aredriven in response to a control signal ENBL_even, for example. The senseamplifiers SA_Bi and the clamp circuit CLAMP_A are driven in response toa control signal ENBL_odd, for example.

With the configuration of the MRAM according to the fifth embodiment, itis possible to selectively connect one of the column blocks CB to thelocal DQ lines LDQ, and read data from or write data to the selectedcolumn block CB.

For example, it is assumed that the column select line CSL(N) selectsthe column block CB(N). In this case, the column select line CSL(N)turns on the transfer gates XFER_A and XFER_B between the column blockCB(N) and the sense amplifiers SA. The bit lines BL0, BL2, BL4, and BL6are thereby connected to the local DQ lines LDQ_A0 to LDQ_A3, and thebit lines BL1, BL3, BL5, and BL7 are thereby connected to the local DQlines LDQ_B0 to LDQ_B3, respectively.

When one of the word lines WL0, WL2, WL4, and the like is selected, thecontrol signal ENBL_even is activated to drive the sense amplifiers SA_Aand the clamp circuit CLAMP_B. The bit lines BL0, BL2, BL4, and BL6 arethereby connected to the sense amplifiers SA_A via the local DQ linesLDQ_A0 to LDQ_A3, respectively. In addition, the bit lines BL1, BL3,BL5, and BL7 are connected to the reference voltage VSS via the local DQlines LDQ_B0 to LDQ_B3, respectively. The sense amplifiers SA_A0 toSA_A3 thereby perform a data read operation.

When one of the word lines WL1, WL3, WL5, and the like is selected, thecontrol signal ENBL_odd is activated to drive the sense amplifiers SA_Band the clamp circuit CLAMP_A. The bit lines BL1, BL3, BL5, and BL7 arethereby connected to the sense amplifiers SA_B via the local DQ linesLDQ_B0 to LDQ_B3, respectively. In addition, the bit lines BL0, BL2,BL4, and BL6 are connected to the reference voltage VSS via the local DQlines LDQ_A0 to LDQ_A3, respectively. The sense amplifiers SA_B0 toSA_B3 thereby perform the data read operation.

In this manner, the sense amplifiers SA_Ai detect the data stored in thememory cells MC connected to the even-number word lines WL, and thesense amplifiers SA_Bi detect the data stored in the memory cells MCconnected to the odd-number word lines WL.

Because a plurality of column blocks CB share the sense amplifiers SA,the total number of the sense amplifiers SA can be decreased. In turn,this can contribute to reducing the chip size of the MRAM, and thisleads to cost reduction.

Sixth Embodiment

FIG. 18 is a schematic diagram showing a configuration example of thememory cell array MCA, local DQ lines LDQ, and the sense amplifiers SAin an MRAM according to a sixth embodiment. The sixth embodiment differsfrom the fifth embodiment in that the memory cell array MCA has the same(staircase) configuration as that of the memory cell array MCA accordingto the second embodiment. The MRAM according to the sixth embodiment canbe configured similarly to that according to the fifth embodiment inother aspects. Therefore, the connection relation between the senseamplifiers SA and the column blocks CB according to the sixth embodimentcan be regarded as the same as that according to the fifth embodiment.

In the sixth embodiment, the bit lines BL to be paired at the time ofthe data read operation or the data write operation change depending onthe selected word line WL. Accordingly, it suffices to change theconnection relation between the column blocks CB and the senseamplifiers SA and that between the column blocks CB and the referencevoltage VSS.

For example, it is assumed that the column select line CSL(N) selectsthe column block CB(N). In this case, the column select line CSL(N)turns on the transfer gates XFER_A and XFER_B between the column blockCB(N) and the sense amplifiers SA.

The bit lines BL1, BL3, BL5, and BL7 are thereby connected to the localDQ lines LDQ_A0 to LDQ_A3, and the bit lines BL0, BL2, BL4, and BL6 arethereby connected to the local DQ lines LDQ_B0 to LDQ_B3, respectively.

When one of the word lines WL0, WL2, WL4, and the like is selected, thecontrol signal ENBL_even is activated to drive the sense amplifiers SA_Aand the clamp circuit CLAMP_B. The bit lines BL1, BL3, BL5, and BL7 arethereby connected to the sense amplifiers SA_A via the local DQ linesLDQ_A0 to LDQ_A3, respectively. In addition, the bit lines BL0, BL2,BL4, and BL6 are connected to the reference voltage VSS via the local DQlines LDQ_B0 to LDQ_B3, respectively. The sense amplifiers SA_A0 toSA_A3 thereby perform a data read operation. At this time, the pairedbit lines BL are (BL1, BL2), (BL3, BL4), (BL5, BL6), and the like. Asfor the unselected column blocks CB, the potentials of the respectivebit lines BL are fixed to the reference voltage VSS or the transfergates XFER_B connected to the unselected column blocks CB are turned on.

When one of the word lines WL1, WL3, WL5, and the like is selected, thecontrol signal ENBL_odd is activated to drive the sense amplifiers SA_Band the clamp circuit CLAMP_A. The bit lines BL1, BL3, BL5, and BL7 arethereby connected to the reference voltage VSS via the local DQ linesLDQ_A0 to LDQ_A3, respectively. In addition, the bit lines BL0, BL2,BL4, and BL6 are connected to the sense amplifiers SA_B via the local DQlines LDQ_B0 to LDQ_B3, respectively. The sense amplifiers SA_B0 toSA_B3 thereby perform a data read operation. At this time, the pairedbit lines BL are (BL0, BL1), (BL2, BL3), (BL4, BL5), and the like. Asfor the unselected column blocks CB, the potentials of the respectivebit lines BL are fixed to the reference voltage VSS or the transfergates XFER_A connected to the unselected column blocks CB are turned on.

Because a plurality of column blocks CB share the sense amplifiers SA,the total number of the sense amplifiers SA can be decreased. In turn,this can contribute to reducing the chip size of the MRAM, and thisleads to cost reduction.

Modification of Fifth Embodiment

FIG. 19 is a schematic diagram showing a configuration example of thememory cell array MCA, the local DQ lines LDQ, the transfer gates XFER,and the sense amplifiers SA in an MRAM according to a modification ofthe fifth embodiment. The MRAM according to the present modification isconfigured to connect one of each pair of bit lines BLi and BLi+1 to thesense amplifier SA via the local DQ line LDQ and to connect the otherbit line BLi or BLi+1 to the reference voltage VSS without via the localDQ line LDQ. The sense amplifiers SA provided on the both sides of thememory cell array MCA can thereby simultaneously read data. For example,to read n-bit data from one column block CB, it suffices to provide n/2sense amplifiers SA on each side of the memory cell array MCA.

The memory cell array MCA can be configured similarly (rectangular waveconfiguration) to that according to the first embodiment.

For example, in FIG. 19, the memory cells MC corresponding to four pairsof bit lines BLi and BLi+1 constitute one column block CB. The two senseamplifiers SA_A and the two sense amplifiers SA_B are arranged on theboth sides of the memory cell array MCA, respectively. The local DQlines LDQ_A and LDQ_B and transfer gates XFER_A and XFER_B are providedbetween the sense amplifiers SA and the column blocks CB.

Each of the transfer gates XFER is connected between one bit line BL andone local DQ line LDQ, and controlled to be turned on or off by thecolumn select line CSL. When the corresponding column block CB isselected, each of the transfer gates XFER connects the bit line BL inthe selected column block CB to the sense amplifier SA via the local DQline LDQ. Alternatively, when the corresponding column block CB isselected, each of the transfer gates XFER connects the bit line BL inthe selected column block CB directly to the reference voltage VSSwithout via the local DQ line LDQ.

The sense amplifiers SA are connected to the corresponding local DQlines LDQ, respectively.

For example, when a column select line CSL_A(N) is activated, the senseamplifier SA_A0 on the right side of the column block CB(N) is connectedto the bit line BL0 via one local DQ line LDQ_A and the sense amplifierSA_A1 is connected to the bit line BL4 via one local DQ line LDQ_A.Furthermore, the sense amplifier SA_B0 on the left side of the columnblock CB(N) is connected to the bit line BL7 via one local DQ lineLDQ_B, and the sense amplifier SA_B1 is connected to the bit line BL3via one local DQ line LDQ_B.

On the other hand, the bit lines BL1 and BL5 are connected to thereference voltage VSS via the transfer gates XFER_A, and the bit linesBL2 and BL6 are connected to the reference voltage VSS via the transfergates XFER_B, respectively.

The sense amplifiers SA_A0, SA_A1, SA_B0, and SA_B1 thereby detect datastored in the memory cells MC via the bit lines BL0, BL4, BL7, and BL3,respectively. That is, the sense amplifiers SA can simultaneously detectfour-bit data.

When a column select line CSL_B(N) is activated, the sense amplifierSA_A0 is connected to the bit line BL2 via one local DQ line LDQ_A andthe sense amplifier SA_A1 is connected to the bit line BL6 via one localDQ line LDQ_A. Furthermore, the sense amplifier SA_B0 is connected tothe bit line BL5 via one local DQ line LDQ_B, and the sense amplifierSA_B1 is connected to the bit line BL1 via one local DQ line LDQ_B.

On the other hand, the bit lines BL0 and BL4 are connected to thereference voltage VSS via the transfer gates XFER_A, and the bit linesBL3 and BL7 are connected to the reference voltage VSS via the transfergates XFER_B, respectively.

The sense amplifiers SA_A0, SA_A1, SA_B0, and SA_B1 thereby detect datastored in the memory cells MC via the bit lines BL2, BL6, BL5, and BL1,respectively. In this case, similarly to the case described above, thesense amplifiers SA can simultaneously detect four-bit data.

For example, in the MRAM, when the even-number word lines WL0, WL2, WL4,and the like are selected, the column select line CSL_A(N) is activatedto read the data. When the odd-number word lines WL1, WL3, WL5, and thelike are selected, the column select line CSL_B(N) is activated to readthe data.

By allowing the MRAM to operate as described above, the cell currentsIcell flow to the memory cells MC in the same direction (A2 in FIG. 2).In the present modification like the modification of the firstembodiment, the read margin can be increased.

According to the present modification, the number of the senseamplifiers SA can be decreased because a plurality of column blocks CBshare the sense amplifiers SA. Moreover, each of the transfer gates XFERselectively connects one of the two bit lines BL to one local DQ lineLDQ. Therefore, all the local DQ lines LDQ arranged on the both sides ofthe column block CB can be connected to the bit lines BL at the time ofthe data read operation and the data write operation. As a result, itsuffices to arrange as many sense amplifiers SA as bits of the data tobe read simultaneously on the both sides of the column block CB. Inturn, this can contribute to reducing the chip size of the MRAM, andthis leads to cost reduction.

Modification of Sixth Embodiment

FIG. 20 is a schematic diagram showing a configuration example of thememory cell array MCA, the local DQ lines LDQ, the transfer gates XFER,and the sense amplifiers SA in an MRAM according to a modification ofthe sixth embodiment. In the present modification, the modification ofthe fifth embodiment is applied to the sixth embodiment.

The modification of the sixth embodiment differs from that of the fifthembodiment in that the memory cell array MCA has the same (staircase)configuration as that of the memory cell array MCA according to thesecond embodiment. The MRAM according to the modification of the sixthembodiment can be configured similarly to that according to themodification of the fifth embodiment in other aspects. Therefore, theconnection relation between the sense amplifiers SA and the columnblocks CB according to the modification of the sixth embodiment can beregarded as the same as that according to the modification of the fifthembodiment.

In the modification of the sixth embodiment, the bit lines BL to bepaired at the time of the data read operation or the data writeoperation change depending on the selected word line WL. Accordingly, itsuffices to change the connection relation between the column blocks CBand the sense amplifiers SA and that between the column blocks CB andthe reference voltage VSS.

For example, when the column select line CSL_A(N) is activated, thesense amplifier SA_A0 is connected to the bit line BL7 via one local DQline LDQ_A and the sense amplifier SA_A1 is connected to the bit lineBL3 via one local DQ line LDQ_A. Furthermore, the sense amplifier SA_B0on the left side of the column block CB(N) is connected to the bit lineBL0 via one local DQ line LDQ_B, and the sense amplifier SA_B1 isconnected to the bit line BL4 via one local DQ line LDQ_B.

On the other hand, the bit lines BL1 and BL5 are connected to thereference voltage VSS via the transfer gates XFER_A, and the bit linesBL2 and BL6 are connected to the reference voltage VSS via the transfergates XFER_B, respectively.

As for the unselected column blocks CB, the potentials of the respectivebit lines BL are fixed to the reference voltage VSS or the transfergates XFER_B connected to the unselected column blocks CB are turned on.

The sense amplifiers SA_A0, SA_A1, SA_B0, and SA_B1 thereby detect datastored in the memory cells MC via the bit lines BL7, BL3, BL0, and BL4,respectively. That is, the sense amplifiers SA can simultaneously detectfour-bit data.

When the column select line CSL_B(N) is activated, the sense amplifierSA_A0 is connected to the bit line BL5 via one local DQ line LDQ_A andthe sense amplifier SA_A1 is connected to the bit line BL1 via one localDQ line LDQ_A. Furthermore, the sense amplifier SA_B0 is connected tothe bit line BL2 via one local DQ line LDQ_B, and the sense amplifierSA_B1 is connected to the bit line BL6 via one local DQ line LDQ_B.

On the other hand, the bit lines BL3 and BL7 are connected to thereference voltage VSS via the transfer gates XFER_A, and the bit linesBL0 and BL4 are connected to the reference voltage VSS via the transfergates XFER_B, respectively.

As for the unselected column blocks CB, the potentials of the respectivebit lines BL are fixed to the reference voltage VSS or the transfergates XFER_B connected to the unselected column blocks CB are turned on.

The sense amplifiers SA_A0, SA_A1, SA_B0, and SA_B1 thereby detect datastored in the memory cells MC via the bit lines BL5, BL1, BL2, and BL6,respectively. In this case, similarly to the case described above, thesense amplifiers SA can simultaneously detect four-bit data.

For example, in the MRAM, when the odd-number word lines WL1, WL3, WL5,and the like are selected, the column select line CSL_A(N) is activatedto read the data. Therefore, the paired bit lines BL are (BL0, BL1),(BL2, BL3), (BL4, BL5), and the like. The sense amplifiers SA can detectthe data via the bit lines BL7, BL3, B0, and BL4.

In the MRAM, when the even-number word lines WL0, WL2, WL4, and the likeare selected, the column select line CSL_B(N) is activated to read thedata. Therefore, the paired bit lines BL are (BL1, BL2), (BL3, BL4),(BL5, BL6), and the like. The sense amplifiers SA can detect the datavia the bit lines BL5, BL1, BL2, and BL6.

The modification of the sixth embodiment can achieve effects identicalto those of the modification of the fifth embodiment.

By allowing the MRAM to operate as described above, the cell currentsIcell flow to the memory cells MC in the same direction (A2 in FIG. 2).In the present modification like the modification of the secondembodiment, the read margin can be increased.

While the data read operation has been mainly described in theembodiments and modifications, it suffices to carry a write current tothe memory cells MC according to the write data as described withreference to FIG. 2.

As the cell transistor CT constituting each memory cell MC, an arbitrarytransistor such as a planar MOSFET (Metal-Oxide Semiconductor FieldEffect Transistor), a so-called FinFET, a Dual-Channel FinFET, a GAA(Gate All Around) transistor, or an Si Nanowire transistor can be used.

Furthermore, while the MRAM has been described in each of theembodiments as an example of the semiconductor storage device, theembodiments are also applicable to a semiconductor storage device suchas a PCRAM or an RRAM using the other resistance storage elements.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel methods and systems describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the methods andsystems described herein may be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the inventions.

1. A semiconductor storage device comprising: a first memory cellcomprising a first storage element and a first switching element, whichare connected in series between a first bit line and a second bit line;a second memory cell comprising a second storage element and a secondswitching element, which are connected in series between the second bitline and a third bit line; and a third memory cell comprising a thirdstorage element and a third switching element, which are connected inseries between the third bit line and a fourth bit line, wherein thesecond bit line is adjacent to the first bit line, wherein the third bitline is adjacent to the second bit line, and wherein the fourth bit lineis adjacent to the third bit line.
 2. The device of claim 1, wherein thefirst switching element is connected to the first bit line and the firststorage element is connected to the second bit line, the secondswitching element is connected to the second bit line and the secondstorage element is connected to the third bit line, the third switchingelement is connected to the third bit line and the third storage elementis connected to the fourth bit line.
 3. The device of claim 1, whereindata is read from or written to the first memory cell via the first bitline and the second bit line when the data is read from or written tothe first memory cell, data is read from or written to the second memorycell via the second bit line and the third bit line when the data isread from or written to the second memory cell, and data is read from orwritten to the third memory cell via the third bit line and the fourthbit line when the data is read from or written to the third memory cell.4. The device of claim 1, wherein the first, second and third storageelements are magnetic tunnel junction elements.
 5. The device of claim1, wherein the first, second and third switching elements aretransistors.
 6. The device of claim 1, wherein the first, second andthird switching elements are in an active area defined by an elementisolation region on a semiconductor substrate, and the first, second andthird memory cells are above the semiconductor substrate.
 7. The deviceof claim 1, wherein the first memory cell, the second memory cell, andthe third memory cell are connected in series.
 8. A semiconductorstorage device comprising: a first memory cell connected between a firstbit line and a second bit line and connected to a first word line; and asecond memory cell connected between the second bit line and a third bitline and connected to a second word line, wherein the second bit line isadjacent to the first bit line, wherein the third bit line is adjacentto the second bit line, wherein the second word line is adjacent to thefirst word line, and wherein, when the first word line is selected in awrite or a read operation, current flows through the first bit line, thefirst memory cell and the second bit line, when the second word line isselected in a write or a read operation, current flows through the thirdbit line, the second memory cell and the second bit line.
 9. The deviceof claim 8, further comprising: at least a sense amplifier sensing datastored in the first or the second memory cell, wherein when the firstword line is selected in a write or a read operation, the senseamplifier or one of the sense amplifiers is electrically connected to apair of the first and the second bit lines, when the second word line isselected in a write or a read operation, the sense amplifier or one ofthe sense amplifiers is electrically connected to a pair of the secondand the third bit lines.
 10. The device of claim 8, wherein the firstmemory cell comprises a first storage element and a first switchingelement, which are connected in series between the first bit line andthe second bit line, the second memory cell comprises a second storageelement and a second switching element, which are connected in seriesbetween the second bit line and the third bit line, an end of the firstswitching element is connected to the first bit line, another end of thefirst switching element is connected to an end of the first storageelement, another end of the first storage element is connected to thesecond bit line, and an end of the second switching element is connectedto the second bit line, another end of the second switching element isconnected to an end of the second storage element, another end of thesecond storage element is connected to the third bit line.
 11. Thedevice of claim 9, wherein one of the sense amplifiers is electricallyconnected to the second bit line, when the first word line is selectedin a write or a read operation, the sense amplifier is connected to thefirst memory cell via the second bit line, when the second word line isselected in a write or a read operation, the sense amplifier isconnected to the second memory cell via the second bit line.
 12. Thedevice of claim 8, wherein, when the first word line is selected in awrite or a read operation, current flows from the second bit line to thefirst bit line through the first memory cell, when the second word lineis selected in a write or a read operation, current flows from thesecond bit line to the third bit line through the second memory cell.13. The device of claim 9, wherein, when the first word line is selectedin a write or a read operation, the second bit line is electricallyconnected to the sense amplifier and the first bit line is electricallyconnected to a current sink, when the second word line is selected in awrite or a read operation, the second bit line is electrically connectedto the sense amplifier and the third bit line is electrically connectedto the current sink.
 14. The device of claim 9, wherein one of two bitlines adjacent to each other is electrically connected to the senseamplifier, and another of the two bit lines is electrically connected tothe current sink.
 15. The device of claim 10, wherein current flows fromthe first storage element to the first switching element in the firstmemory cell, and current flows from the second switching element to thesecond storage element in the second memory cell.
 16. The device ofclaim 8, further comprising: at least a sense amplifier sensing datastored in the first or the second memory cell, wherein a first senseamplifier of the sense amplifiers corresponds to the first bit line, asecond sense amplifier of the sense amplifiers corresponds to the secondbit line, when the first word line is selected in a write or a readoperation, the second sense amplifier is electrically connected to thesecond bit line, when the second word line is selected in a write or aread operation, the first sense amplifier is electrically connected tothe first bit line.
 17. The device of claim 16, wherein when the firstword line is selected in a write or a read operation, the first bit lineis electrically connected to a current sink, when the second word lineis selected in a write or a read operation, the second bit line thesecond sense amplifier is electrically connected to the current sink.18. The device of claim 10, wherein, when the first word line isselected in a write or a read operation, current flows from the firstbit line to the second bit line through the first memory cell, when thesecond word line is selected in a write or a read operation, currentflows from the second bit line to the third bit line through the secondmemory cell.
 19. The device of claim 10, wherein current flows from thefirst switching element to the first storage element in the first memorycell, and current flows from the second switching element to the secondstorage element in the second memory cell.
 20. The device of claim 8,wherein the first memory cell comprises a first storage element and afirst switching element, the second memory cell comprises a secondstorage element and a second switching element, the first storageelement and the first switching element are located under the first bitline, the second storage element and the second switching element arelocated under the second bit line, the first switching element iselectrically connected to the first bit line via contact plugs which arelocated one above the other, the first storage element is electricallyconnected to the second bit line via a wiring which extends in adirection perpendicular to the second bit line in a planer layout, thesecond switching element is electrically connected to the second bitline via a contact plug, the second storage element is electricallyconnected to the third bit line via a wiring.
 21. The device of claim 9,further comprising: a transfer gate part connecting the sense amplifiersto one of bit line blocks, each bit line block comprising a plurality ofthe bit lines, wherein the sense amplifiers are shared by a plurality ofthe bit line blocks, and the transfer gate part electrically connectsthe sense amplifiers to one of the bit line blocks in a write or a readoperation.
 22. The device of claim 21, wherein, when the first word lineis selected in a write or a read operation, the transfer gate partelectrically connects one of the sense amplifiers to a pair of the firstand the second bit lines in one of the bit line blocks, when the secondword line is selected in a write or a read operation, the transfer gatepart electrically connects one of the sense amplifiers to a pair of thesecond and the third bit lines in one of the bit line blocks.
 23. Thedevice of claim 21, wherein the number of the sense amplifiers isidentical to the number of the bit lines in one of the bit line blocks.24. The device of claim 21, wherein the number of the sense amplifiersis identical to the number of pairs of the bit lines in one of the bitline blocks.